Device-to-Device (D2D) communication as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, but the concept of allowing local D2D communication to use cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. The non-orthogonal resource sharing between the cellular network and the D2D layers has the potential of reuse gain and proximity gain while at the same time increasing the resource utilization. Because of this, D2D communication underlying cellular networks has received considerable interest in the recent years.
Specifically, in 3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) networks, such as LTE Direct, D2D communication can be used in commercial applications, such as cellular network offloading, proximity based social networking, or in public safety situations in which first responders need to communicate with each other and with people in the disaster area. For more details, the interested reader is directed to 3GPP Technical Report (TR) 22.803. This has led to plans to standardize User Equipment (UE) capability in LTE for supporting D2D communication, especially in the National Security Public Safety (NSPS) network.
D2D communication entities using a LTE Direct link may reuse the same Physical Resource Blocks (PRB) as used for cellular communications either in the downlink, in the uplink, or both. The PRB refer to the time and/or frequency resources to be used. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency, but at the expense of some increase of the intra-cell interference. In many implementations, D2D communicating entities use uplink resources such as uplink PRBs but it is possible that D2D communications take place in the cellular downlink spectrum in a Frequency Division Duplexing (FDD) system or in downlink time slots in a cellular Time Division Duplexing (TDD) system. The term D2D communication comprises, but is not limited to, transmission and/or reception of signals such as synchronization signals, discovery signals, reference signals, broadcast information, control information, payload data, etc.
When a device, such as a User Equipment (UE), is turned on, it starts to look for a “good enough” Radio Access Technology (RAT) to camp on. As used herein, camping means that the device has completed the cell selection/reselection process and has chosen a cell. The device monitors system information and (in most cases) paging information. The device might monitor several frequency bands and carrier frequencies in order to find a suitable cell. Once a suitable cell is detected (typically strong enough to be able to read broadcast information, and the device is allowed to camp on that cell), the device registers to the network and starts monitoring the carrier for paging messages from the camping cell. In case the strongest detected cell on one carrier is worse than a threshold, the device enables Inter-RAT (IRAT) measurements (searching for cells on other carriers that may support a different RAT), and does reselection to another detected RAT, if that carrier/RAT is “good enough.” The device only does IRAT measurements when necessary, and the search is only performed on radio signals transmitted by base stations in downlink resources (frequency/time).
When a device finds a suitable cell to camp on, the device may not detect D2D communications that are available. Therefore, there is a need for improved use of D2D communications.